1. Technical Field of the Invention
The present invention relates generally to generators and electric power generation systems for motor vehicles. More particularly, the invention relates to a control apparatus for a vehicle generator, which provides information about operation of the generator to an external control apparatus using a PWM (Pulse Width Modulated) signal that has a duty and a frequency both of which are specially determined.
2. Description of the Related Art
Several methods have been proposed to provide an external control apparatus with information about operation of a vehicle generator, such as fault conditions of the generator.
For example, Japanese Patent First Publication No. 2002-10515 discloses a control apparatus for a vehicle generator (or alternator), which is configured to send, via a single signal line, to an external control apparatus (or unit) both a first signal indicative of a duty of a transistor for controlling a field current supply to the generator and a second signal indicative of a fault condition (abnormality occurrence status) with the generator. Specifically, the control apparatus sends to the external control apparatus the first signal when the generator is in normal condition and the second signal when it is in the fault condition.
However, the above control apparatus cannot concurrently inform the external control apparatus of the duty of the transistor and the fault condition of the generator with a single signal, since the second signal is configured to have a low level. Further, the electric potential of the signal line will drop to a low level when the signal line is short-circuited to the ground. Thus, it is impossible for the external control apparatus to distinguish the fault condition of the generator from that of the signal line.
Japanese Patent First Publication No. H11-146698 discloses a control apparatus for a vehicle generator, which is configured to send, via a single signal line, to an external control apparatus (or vehicle ECU) a PWM signal that is indicative of both a duty of a transistor for controlling a field current supply to the generator and a rotational speed of the generator. Specifically, the PWM signal has a duty determined as a function of the duty of the transistor and a frequency determined as a function of the rotational speed of the generator.
However, in the above case, the frequency of the PWM signal varies with the rotational speed of the generator. Consequently, when a PWM command signal, which is indicative of a command for control of the generator determined based on the PWM signal, is sent from the external control apparatus to the control apparatus, there will be a difference in frequency between the two PWM signals that varies with time. As a result, the relationship between the reception time of the PWM signal and transmission time of the PWM command signal for the external control apparatus will also vary with time, thus causing instability of the whole electric power generation system of the vehicle.
Further, the control apparatus is also configured to send to the external control apparatus a high level or a low level signal when the generator is in fault conditions. However, the electric potential of the signal line will also become high or low when the signal line is open-circuited or short-circuited. Thus, it is impossible for the external control apparatus to distinguish fault conditions of the generator from those of the signal line.
Japanese Patent First Publication No. S63-206125 discloses a control apparatus for a vehicle generator, which is configured to alternately energize a charge lamp and a choke heater by allowing a switch to have a prescribed duty when the generator is in fault conditions. With such a configuration, the choke heater is allowed to be supplied with electric power even when the generator is in fault conditions, thus preventing malfunction of the vehicle engine due to any stop of electric power supply to the choke heater.
However, the above control apparatus informs all the fault conditions of the generator only through the flashing on and off of the charge lamp, so it is impossible to identify different types of fault conditions. Further, the fault conditions of the generator are not informed to an external control apparatus, which may, otherwise, perform suitable vehicle controls based on the different types of fault conditions.